WYWyoming Geography

On the Two Ocean Plateau in the Teton Wilderness, a creek comes down off the divide and splits at a low sill of rock. The left branch is Atlantic Creek, which joins the Yellowstone, then the Missouri, then the Mississippi, and reaches the Gulf of Mexico. The right branch is Pacific Creek, which joins the Snake and the Columbia and reaches the sea at Astoria. The Forest Service marks the spot the Parting of the Waters. A trout can swim across the Continental Divide there without leaving the water.

Water leaves in four directions, or never leaves at all

Wyoming feeds more major river systems than almost any comparable piece of North America. The Snake heads for the Columbia and the Pacific. The Green heads for the Colorado and the Gulf of California. The North Platte, Bighorn, Powder and Yellowstone all run into the Missouri and end in the Gulf of Mexico. The Bear River, in the southwest corner, drains into the Great Salt Lake and reaches no ocean at all.

Then there is the Great Divide Basin. South of the Wind River Range the Continental Divide does something it does nowhere else in the country: it forks, runs around both sides of a bowl roughly 4,000 square miles across, and rejoins on the far side. Nothing drains out. Water landing on the Red Desert inside it evaporates or sinks, and the ground holds the Killpecker Dunes, one of the largest active dune fields in the country, with ice preserved under some of the sand year round.

One river here changes name in mid-course. The Wind River flows north through Wind River Canyon and emerges as the Bighorn, at a spot near Thermopolis signposted the Wedding of the Waters. Two parties of explorers named the same water from opposite ends and neither gave in.

A rectangle that is not quite a rectangle

Wyoming and Colorado are the only states whose borders are defined entirely by lines of latitude and longitude, with no river, ridge or coastline anywhere in them. Wyoming runs from 41 to 45 degrees north and from 104 degrees 3 minutes to 111 degrees 3 minutes west: four degrees by seven. On a sphere that is not a rectangle. Meridians converge toward the pole, so the northern edge is shorter than the southern one and the state is really a trapezoid, about 365 miles across and 276 miles from top to bottom.

The ground is messier than the paper. Crews working in the 1870s with chains and transits laid the boundary in segments that do not quite meet, and the legal border is the one they marked, kinks included. Six states touch it: Montana, South Dakota, Nebraska, Colorado, Utah and Idaho.

Gannett Peak, and a low point higher than most states' summits

Gannett Peak straddles the Fremont and Sublette county line in the Wind River Range at 13,809 feet, and it carries the largest glaciers in the American Rockies. Reaching it means walking more than 20 miles from the nearest road, which makes it the most remote state high point in the lower 48.

The low point is about 3,100 feet, where the Belle Fourche River crosses into South Dakota. That floor is higher than the highest ground in more than a dozen other states, Ohio, Michigan, Wisconsin and Alabama among them. Only Colorado averages higher than Wyoming's roughly 6,700 feet.

The Tetons rise on a different schedule. The range is among the youngest in the Rockies, lifted along the Teton fault starting perhaps 10 million years ago, and it has no foothills. Jackson Hole sits near 6,800 feet and Grand Teton reaches 13,775, so the wall goes up close to 7,000 feet in one step.

Ninety six percent of Yellowstone

Yellowstone was drawn in 1872 around a feature nobody then understood: a caldera roughly 30 by 45 miles wide, the collapsed roof of a magma system fed by a hotspot the continent has been sliding across for about 16 million years. Of the park's 2.2 million acres, about 96 percent lie in Wyoming, with thin slices in Montana and Idaho.

The plumbing surfaces everywhere. More than 10,000 hydrothermal features sit inside the park, including roughly half the geysers on earth. Grand Prismatic Spring measures about 370 feet across, the largest hot spring in the country, its rings of colour made by heat-loving microbes sorted by temperature. Yellowstone Lake covers 136 square miles at 7,732 feet, the largest lake above 7,000 feet in North America, with hydrothermal vents on its floor.

Below the lake the Yellowstone River drops over two waterfalls into a canyon up to 1,200 feet deep, its walls yellow because hot water altered the rhyolite. That colour named the river, and the river named the park.

South Pass, the Gangplank, and the wind

Wyoming is where the Rockies are easiest to cross, which is why the emigrant trails and later the railway came through. South Pass, 7,412 feet on the south end of the Wind River Range, is a saddle so broad and gently graded that wagon parties crossed the Continental Divide without being sure they had. Hundreds of thousands on the Oregon, California and Mormon trails went over it.

Further south the Union Pacific found the Gangplank west of Cheyenne, a ramp of old sediment leaning against the Laramie Mountains that carries a railway from the High Plains to 8,000 feet on a steady grade with no switchbacks. Interstate 80 still uses it, and pays for the open terrain in wind: the stretch between Laramie and Rawlins closes to high-profile vehicles routinely each winter.

The record high is 115 degrees at Basin on July 12, 1900. The record low is 66 below zero at Riverside on February 9, 1933. In the northeast corner, Devils Tower stands 867 feet above its base, a column of igneous rock left behind when softer sediment eroded away, and became the first national monument in the country on September 24, 1906. All of Wyoming keeps Mountain Time.

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